Welcome to a journey through the fascinating world of electronic tubes, true gems of vintage technology that have left an indelible mark on the history of audio amplification. In this article, we will explore the EL84 family, one of the most iconic and versatile tubes ever produced. Since its introduction by Mullard in 1954, the EL84 has won over audiophiles with its excellent performance and the warm, enveloping sound it can deliver. We will not only examine the characteristics of the EL84, but also take a look at its variants such as UL84, PL84, and 7189, discovering how these tubes have influenced the landscape of amplification. In addition, we will make a surprising foray into the world of the EL86, a close relative of the EL84 that shares many of its distinctive characteristics. I am writing this article with the aim of dispelling the confusion that often surrounds the designations of these tube variants, offering clarity and understanding in the magical world of electronic tubes.
The EL84
The EL84 power pentode was introduced by Mullard in 1954. To demonstrate its use and quality, Mullard’s applications laboratory produced an amplifier design that later became a classic known as the “Mullard 5-10“. When the EL84 is used in an ultra-linear push-pull configuration with a 43% distributed load, it can achieve an output power of 10 W with a total harmonic distortion of 0.9% in class A. In class AB configuration, it is possible to reach power levels of up to 15 watts. In the photo, a NOS EL84 branded Zenith is shown next to a pair of custom-made output transformers designed to fully enhance the performance of this tube.
Confusion in Designations: Differences Between EL84, UL84, and Other Variants
In general, the first letter in a tube designation, such as the “E” in “EL84”, indicates the heater voltage, which in the specific case of the EL84 is 6.3 volts. According to the classical convention applied to all other tubes, one would expect a “UL84” to be essentially identical to the EL84, but with a different heater voltage. However, this expectation does not match reality.
When the UL84 was created, the goal was to develop a tube dedicated to radios in which all tubes had their heaters wired in series. These radios were generally powered by autotransformers and, for various reasons that I will not list here, required an audio output tube that operated at lower voltages than the EL84. As a result, not only was the heater modified to operate at 45 volts with a current of 100 mA, but the anode structure was also changed, making it smaller and suitable for operation at lower voltages, along with modifications to the screen grid. This led to the creation of a tube that is different from the EL84, breaking the tube nomenclature standards that were in place at the time.
Later, for various reasons, the different electrical characteristics of the UL84 also proved useful for other applications. Thus, the PL84 variant was created, intended for use in televisions with a 15-volt, 300 mA heater. Finally, a version with a 6.3-volt heater was desired, and it was named EL86. In summary, UL84 = PL84 = EL86 (with only different heaters), while these three tubes differ completely from the EL84 in all other respects.
The creation of the UL84, PL84, and EL86 variants added a layer of confusion to tube nomenclature, breaking with the previously followed standard. This departure from tradition generated uncertainty, as one would have expected tubes with similar names to be substantially equivalent, differing only in heater voltage. Interestingly, many DIY builders look for the EL86 without knowing that, apart from the heater voltage, it is the same tube as the UL84 and the PL84.
UL84 – PL84 and EL86 curves at 170 volts screen grid (also available in the datasheet)
UL84 – PL84 and EL86 curves connected as triode
6N43N-E, a little-known tube
The 6N43N-E, a Russian variant compatible with the EL86, is mainly distinguished by its internal construction: it is a beam tetrode rather than a pentode like the EL86. However, since the datasheet for this tube does not include curves with G2 biased at 170 V, which would be necessary for a direct comparison with the EL86, the curves were acquired using a u-tracer and published for this purpose.
G2 +100 V
G2 +170 V
6N43N-E connected as triode
When the EL84 Becomes a Killer: The Truth About the 7189
The 7189 represents a variant of the EL84, but with higher plate and screen grid voltage ratings compared to the standard EL84. This configuration allows the 7189 to deliver higher output power, making it a common choice in many hi-fi amplifier applications in the late 1950s and early 1960s.
It is important to emphasize that, despite the similarity between the two tube types, the 7189 is not equivalent to the EL84. This distinction is crucial to avoid installation errors and damage to equipment. Some sellers on platforms such as eBay incorrectly advertise the 7189 as equivalent to the EL84, leading buyers to install the latter in devices designed for the 7189. This confusion is so widespread that most online sellers, with the exception of a few reliable ones, tend to present them as interchangeable and offer them as if they were identical. This practice can unfortunately mislead buyers, sometimes unknowingly, resulting in situations that amount to fraud.
There are significant differences between EL84 (or 6BQ5) tubes and the 7189. The maximum anode voltage of the EL84 is 300 V, while the 7189 can withstand higher anode voltages, typically up to 440 V. The 7189 is a reinforced version of the EL84 capable of handling higher voltages, thus allowing greater output power. However, it is important to note that these two tubes are not equivalent. An EL84 installed in a circuit designed for the 7189 would be subjected to anode and screen grid voltages beyond its operational limits. This scenario often results in overcurrent conditions or tube failure, which can easily damage the primary winding of the output transformer. Let us look at the photo below:
In the center is an EL84, operating at an anode voltage of 300 volts. On the right is a UL84, a version with a 45-volt heater designed to operate with a maximum anode voltage of 170 volts. It is easy to see that the anode structure of the UL84 is narrower than that of the EL84. Finally, on the left is a 7189A, highlighted by its massive anode structure, which is significantly larger than that of the EL84.
Small equivalence table:
EL84 = 6BQ5 = 6N14N
7189 / 7189A = 6BQ5B
Until recently, 7189 tubes were mainly available as NOS (New Old Stock) items, often at prohibitive prices. However, the good news is that today some manufacturers have started reproducing them, providing an affordable replacement option. This is positive news for owners of vintage amplifiers, as people often tend to replace 7189 tubes with EL84s, with disastrous consequences. In the photo below, a quartet of 7189 tubes from my current favorite manufacturer, Thung-Sol.
Considerations on using EL84 tubes connected as triodes
In the world of tubes, the EL84 and its variants have always attracted great interest for their versatile use in many audio circuits. However, when it comes to using the EL84 connected as a triode, there are some important considerations to keep in mind. While the various EL86 tubes connected as triodes show well-defined and usable curves, the EL84 connected as a triode presents a significant challenge. In an illustrative graph, one can observe a very steep slope in its curves, compromising its suitability for this type of configuration.
Optimistically, even with a dissipation of 12 watts, it is possible to obtain less than half a watt from an EL84 connected as a triode, with a transfer efficiency below 5%. This makes it unsuitable for such applications. It is important to stress that misleading information is not uncommon on the market, with some experts promising EL84 triode amplifiers capable of producing tens of watts. In reality, practical experience often shows a real output of only 0.39 watts RMS in some cases.